The invention relates generally to a video signal processing system for processing a wide bandwidth video signal into a reduced bandwidth signal suitable for transmission and/or recording via a narrow bandwidth signal medium, whereby the information content of the wide bandwidth video signal is retained in the reduced bandwidth signal and the reduced bandwidth signal is compatible with conventional narrow bandwidth reception apparatus, and for receiving and/or reproducing and processing the transmitted reduced bandwidth signal for recovering therefrom the information content of the original wide bandwidth signal. The invention in certain of its aspects relates more particularly to signal processing useful in a narrow bandwidth format video cassette recorder (VCR), for converting a wide bandwidth input video signal to a reduced bandwidth video signal containing the information content of the input wide bandwidth video signal within the reduced bandwidth, whereby the reduced bandwidth video signal may be recorded and reproduced conventionally by such narrow bandwidth format VCR. The invention in other of its aspects relates more particularly to processing the reproduced narrow bandwidth video signal to recover the information content of the wide bandwidth video signal therefrom, whereby a wide bandwidth video signal may be reconstructed for yielding improved video bandwidth of the reproduced signal comparable to the full bandwidth of the input video signal, while maintaining backward compatibility of the recorded reduced bandwidth video signal for playing back video cassettes recorded by this improved video signal processing system on available conventional narrow bandwidth format VCRs or video cassette players (VCPs).
Conventional consumer type VCRs record video information onto video tape cassettes in one of several formats. The well-known VHS format system uses a relatively narrow bandwidth format and produces degraded picture quality in comparison to standard broadcast video chiefly because the recorded VHS format video signal has insufficient horizontal resolution. An enhanced VHS format type recording system, popularly called Super VHS or S-VHS, produces enhanced picture quality by recording a wider bandwidth video signal on the video tape cassette using a higher FM carrier frequency for the luminance information, thus yielding improved picture resolution. Such a format requires a higher FM carrier frequency, higher quality tape in the cassette and higher quality recording and playback mechanisms, heads and circuitry. However, the S-VHS format is not backward compatible with standard VHS format VCRs. I. e., although an S-VHS format VCR can reproduce (playback) cassettes recorded on either S-VHS format or standard VHS format VCRS, a standard VHS format VCR or VCP cannot play back cassettes recorded on S-VHS format VCRS.
It is desirable that an improved video recording system be able to record wider bandwidth video signals on a standard quality cassette than those recordable by conventional narrow bandwidth VCRs, while still maintaining backward compatibility with conventional narrow bandwidth VCRs, and not require especially high quality magnetic tape or record and playback mechanisms. That is, it is desirable that normal-quality, narrow-bandwidth recording tape medium video cassettes may be recordable with wider-bandwidth, higher-frequency video information using the improved system and be able to be compatibly played back by conventional narrow bandwidth VCRs without producing noticeable visual artifacts in the reproduced image, even if the conventional VCR may not be able to reproduce the full-bandwidth signal recorded on such a cassette.
It has long been a goal of video engineers to increase the amount of information transmitted through a given narrowband channel, such as an NTSC signal channel, which is limited to a nominal 4.2 MHz of useful bandwidth. Because the frame and line rates (temporal and vertical resolution) usually are fixed, restricting the bandwidth translates into restricting the horizontal resolution. In some cases, the nominal bandwidth of the channel is limited to 3 MHz or even 2.5 MHz, resulting in an image with insufficient horizontal resolution.
It has long been recognized that in scanned television systems, the signal energy is concentrated spectrally in the spatio-temporal domain at periodic intervals according to the scanning frequencies, and the video spectrum has so-called "holes" or gaps between these discrete signal areas. In these gaps, which gaps also occur at regular intervals, the signal energy is generally comparatively small. The NTSC composite (i. e., "colorplexed") color video system represents a system which uses one of these "holes" to carry the color information. In the NTSC system, the chrominance or "chroma" signal containing the color information is transmitted combined with the baseband video as a pair of color-difference or mixture signals encoded in quadrature amplitude modulation of a suppressed nominally 3.58 MHz color subcarrier. I. e., the color-difference or mixture signals are encoded in respective amplitude-modulation sidebands of a pair of in-phase and quadrature color subcarriers, both of which subcarriers are suppressed. The frequency of the color subcarrier (3.579545 MHz, which is 227.5 times the horizontal scanning frequency of 15.734 kHz) was very carefully selected so that a minimum disturbance occurs when a color video signal is displayed on a black-and-white receiver. Specifically, the NTSC color subcarrier frequency is interleaved horizontally, vertically, and temporally with the luminance signal spectrum to minimize crosstalk and intermodulation between the luminance and chrominance components of the composite video signal.
It was recognized at around the time of the adoption of the NTSC colorplexed system that such frequency spectrum "holes" could also be used to transmit additional horizontal information to increase the horizontal resolution of the reproduced image. In such systems, the higher frequency horizontal information is interleaved spectrally with the lower frequency horizontal information in a similar manner as the chrominance information is in the NTSC color system. An article entitled "REDUCTION OF TELEVISION BANDWIDTH BY FREQUENCY INTERLACE" by E. A. Howson and D. A. Bell in Journal of the British I. R. E., February, 1960, pp. 127-136 contains a description of such a system which utilizes analog signal processing techniques. The artifacts resulting from the frequency interleaving manifest themselves as annoying dot crawl patterns in such a system, so it does not accurately reproduce the full-bandwidth image in its original form.
German Patent Publication No. 82100286.2 entitled "Verfahren zum Ubertagen von Fernsehsignalen uber einen genormten bandbreitebegrenzten Ubertragunskanal und Anordnung zum Durchfuhren des Verfahrens," by Professor Wendland et alii describes principles of offset subsampling and bandwidth compression as applied to advanced television systems. This Jan. 1, 1982, patent publication also describes techniques for implementing television systems in accordance with the principles described therein. Sub-Nyquist sampling is done by replacing every odd sample in a first video line with a zero-valued sample, and then on the next line, replacing every even sample with a zero-valued sample. On alternate frames, the patterns are reversed. A television system using sub-Nyquist sampling at the transmitter can use comb filters in the receivers to suppress dot crawl patterns arising from spectrum interleaving.
Theoretically, the Howson and Bell frequency folding technique and the sub-Nyquist sampling technique are equivalent, when a folding carrier frequency 2f.sub.F used during spectrum reversal is one-half the sampling frequency f.sub.S. The sampled-data digital systems using the sub-Nyquist sampling technique provided improved reconstruction of the received image because of the existence of line and frame combing techniques, which had not been developed at the time of the Howson and Bell system. The sub-Nyquist sampling techniques, however, were developed for totally sampled data digital systems as data reduction (i. e., compression) techniques in digital systems, and the signals generated by these sampling techniques were riot generally intended to be passed through a narrowband analog channel.
An article, "DEVELOPMENT OF HDTV RECEIVING EQUIPMENT BASED ON BAND COMPRESSION TECHNIQUE (MUSE)", by Kojima et alii in IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 4, November 1986, pp.759-768, describes another data compression scheme in which compresses bandwidth by sampling every other pixel each frame, but sampling each pixel once every other frame.
This scheme works well only for non-moving images. For moving images, a motion vector is developed, and the actual rate of sampling of each pixel is adaptively varied in response to the motion vector so that a sample of the pixel is transmitted every other frame on the average, but more often when that pixel is representing a moving image.
U.S. Pat. No. 4,831,463 issued May 16, 1989 to Y. C. Faroudja and entitled VIDEO PROCESSING IN WHICH HIGH FREQUENCY LUMINANCE COMPONENTS ARE FOLDED INTO A MIDBAND SPECTRUM describes apparatus for processing a video signal having a predetermined bandwidth in order to pass the video information contained therewithin through a limited-bandwidth channel, such as magnetic tape. The Faroudja apparatus takes advantage of the fact that the luminance and chrominance components of a composite video signal are separately recorded on electromagnetic tape in a recording system of the color-under type such as the VHS recording system. In a tape recording system of the color-under type the chrominance components separated from the composite video signal are down-converted in frequency to form a color-under signal recorded directly on the tape; and the luminance components separated from the composite video signal are used to modulate the frequency of a luma carrier recorded on the tape as a bias frequency. In the apparatus described in this patent, a video signal preprocessor includes a comb filter to remove remnant energy in the spectral holes between spectrally active areas in the luminance signal spectrum that would be occupied by chrominance in the NTSC composite video signal. A folding circuit then folds the baseband video luminance signal about a predetermined folding frequency f.sub.F selected so that aliases of the baseband luminance signal, generated by heterodyning it with a 2f.sub.F folding carrier at twice the frequency of the f.sub.F folding frequency, are placed into the spectral holes left after the chrominance separation and comb filtering. A lowpass filter then filters the resulting folded video signal so that its bandwidth is about one-half the bandwidth of the original video signal. The resulting signal may then be transmitted through the limited-bandwidth channel.
The Faroudja '463 patent further describes a post-processor which receives the folded signal from the limited-bandwidth channel. The post-processor includes an unfolding circuit which unfolds the received signal about a predetermined unfolding frequency f.sub.U, which is the same as the folding frequency f.sub.F. A comb filter then processes the unfolded signal to remove the alias components resulting from the unfolding process. The signal produced by this comb filter closely approximates the original video signal in terms of the bandwidth and information content.
It is interesting to note that the Howson and Bell article describes two bandwidth reduction techniques for video luminance signals by frequency interlacing or interleaving. In a first technique described, the video luminance signal spectrum is divided into two equal half-bands (i.e., band-split at frequency f.sub.F), and the upper half-band (i.e., the highband luminance from frequency f.sub.F to frequency 2f.sub.F) is used to modulate a sub-carrier which has its frequency set to be near the upper frequency limit of the normal video band (i. e., near 2f.sub.F). The lower sideband of the modulator output is selected and combined with the original lower half-band. The frequency-interlaced signal resulting from such combining contains all of the original luminance signal information, but in one-half the bandwidth of the original signal, and is therefore suitable for transmission over a reduced bandwidth channel.
In a second technique described by Howson and Bell, instead of dividing the main video luminance signal into two half-bands and modulating the 2f.sub.F sub-carrier with the high-frequency half-band only, the entire main video (i. e., baseband) luminance signal is used to modulate the 2f.sub.F sub-carrier. The lower sideband of the modulator output signal contains the required interleaved signal in correct frequency relationship with the main baseband video signal. If the modulator output is added to the main signal and the resultant added signal is passed through a lowpass filter having its break frequency at approximately one-half the sub-carrier frequency, the lowpass filtered output signal consists of the correct composite reduced bandwidth signal (with the sub-carrier suppressed). Howson and Bell teach that this second technique avoids the need for using complementary lowpass and bandpass filters as required by the first technique employing band-splitting. Howson and Bell adopted this second technique in a described experimental apparatus, though the summary abstract appearing in the Howson and Bell article implies that the first technique using band-splitting was employed.
The folding/unfolding system described in the Faroudja '463 patent is similar in principle to the second technique described and adopted by Howson and Bell, in the following regards. Howson and Bell selected the folding modulation sub-carrier frequency to be an odd multiple of one-half the line scan frequency. In Faroudja '463 the frequency of the folding heterodyne oscillator/mixer, or of the sub-Nyquist sampling clock applied to the multiplier used as the folding modulator, is selected to be a harmonic of an odd multiple of the line and frame scan rate. Any harmonic of an odd multiple of the line and frame scan rate is an odd multiple of one-half the line scan frequency, supposing there to be an odd number of scan lines in each frame. In both systems the folding modulation is performed on the entire baseband luminance signal and lowpass filtering is employed after folding to remove frequencies greater than one-half the folding frequency from the folded signal.
In Howson and Bell the high-band luminance is folded into spectral "holes" available because the television signals are luminance-only signals for black-and-white television. In Faroudja '463 the high-band luminance is folded into spectral "holes" from which the NTSC chrominance sub-carrier has been removed. However, because there may still be residual chroma sidebands present in those areas which might interfere with the folding and unfolding processes, the inventors have found it to be preferable instead to fold the high-band luma into the spectral "holes" described by Fukinuki. Then, any residual chroma components when unfolded will be in complementary phase on successive fields and will be optically canceled in the display monitor. In order to fold the high-band video signal into the same band as the low-band video signal, so as to occupy the Fukinuki portions of the band, the folding carrier is chosen to be a harmonic of an even multiple of both the line and the frame scan rates, which harmonic reverses phase from scan line to scan line and from frame to frame. That is, the phase of the folding carrier is reversed at the scan line rate, each reversal being at a respective instant between scan lines.
Both the Howson and Bell article and the Faroudja '463 patent describe folding systems which, if incorporated into an improved VCR, produce cassettes which cannot be played back on conventional VCRs without introducing unacceptable artifacts into the displayed image. This is primarily due to the amplitude of the folded high-frequency components present within the spectrum of the low-frequency components on the previously recorded cassette. The magnitude of the folded high-frequency components is sufficiently high as to introduce intolerable artifacts and degradation (dot crawl, twinkling, line flicker, etc.) into an image display produced from a video signal from which the folded high-frequency components are not properly removed.
Howson and Bell were not particularly concerned with backward compatibility of the interleaved signal. In fact, they suggested including a pre-emphasis filter for boosting the interleaved high-frequency components of the folded luminance signal in order to minimize the effects of crosstalk from the low-frequency luminance components during the transmission of the folded signal through the channel and to minimize sub-carrier interference at the receiver. If a video cassette recorded by a VHS format VCR modified to include the system taught by Howson and Bell were played back on a standard VHS format VCR, the pre-emphasized high-frequency components would produce an even more objectionable image on the television screen than that produced by the Faroudja system.
Thus, the need has remained for improving the video resolution over that available with the currently used, limited-bandwidth video recording and playback techniques, media and mechanisms in a manner which retains backward playback compatibility with existing VCRs and VCPs. Backward playback compatibility is greatly improved by lowering the amplitude of the high-frequency portion of the luminance signal, before or during its being folded into the low-frequency component, a technique called "deemphasis". The deemphasized folded luminance signal is then combined with the chrominance signal and the combined signal recorded on the video tape. Upon playback, the folded deemphasized luminance signal is separated from the chrominance signal. The separated luminance signal is then unfolded, and the amplitude of the high-frequency portion is increased to restore it to substantially its original level, a technique called "reemphasis". If such a video tape is played back on a VCR which does not have the unfolding and reemphasis circuitry, the artifacts caused by the presence of the high-frequency portion of the luminance signal within the low-frequency portion can be reduced to levels that are not objectionable. This is provided for by reducing the amplitude of the high-frequency portion of the luminance signal, which causes the artifacts, to a reduced level either prior to the folding procedure or during the folding procedure.
U.S. Pat. No. 5,113,262 describes a video signal recording system which includes an adaptive deemphasis circuit in the luminance signal record path and an adaptive reemphasis circuit in the playback path. The adaptive deemphasis circuit in U.S. Pat. No. 5,113,262 includes circuitry for detecting the level of the high-frequency portion of the luminance signal, and circuitry for variably reducing the level of the high-frequency portion in response to the detected signal level. If the level of the high-frequency portion of the luminance signal is high, then the level of the high-frequency portion is reduced by a maximum amount; if the level is low, then the level is reduced by a minimum amount. This operation can be referred to as "compression of the dynamic range of luminance signal high frequencies".
A problem that has subsequently been noted by the inventors with regard to the adaptive deemphasis circuit described in U.S. Pat. No. 5,113,262 arises when recording composite video signals that have accompanying noise in the upper luminance frequencies. The circuitry for detecting the level of the high-frequency portion of the luminance signal detects the noise is detected, and the level of the high-frequency portion is reduced in response to the detected noise. This undesirably wipes out low-level high-frequency luminance detail, causing textured surfaces to appear smooth. This problem is solved in an improved adaptive deemphasis circuit that embodies an aspect of the invention. In this improvement a threshold circuit is introduced after the detector that detects the amplitude of the high-frequency portion of the luminance signal component. This threshold circuit operates as a corer, or base-line clipper. Accordingly, the high-frequency portion of the luminance signal is not deemphasized until its amplitude exceeds a threshold level. This threshold level is set to be above the level of noise in the high-frequency portion of the luminance signal component of the composite video signal received for recording.
An adaptive deemphasis circuit and control signal generator constructed in accordance with this aspect of the invention can also be useful in implementing the video tape recording and playback systems described in U.S. patent application Ser. No. 008,813 filed Jan. 25, 1993 by Christopher H. Strolle and Raymond A. Schnitzler, and entitled ADAPTIVE DEEMPHASIS AND REEMPHASIS OF HIGH FREQUENCIES IN VIDEO TAPE RECORDING, UTILIZING A RECORDED CONTROL SIGNAL. U.S. patent application Ser. No. 008,813 is a continuation-in-part of U.S. patent application Ser. No. 604,494 filed Oct. 26, 1990 by Strolle and Schnitzler, and entitled ADAPTIVE DEEMPHASIS AND REEMPHASIS OF HIGH FREQUENCIES IN A VIDEO SIGNAL UTILIZING A RECORDED CONTROL SIGNAL. The inventions described and claimed in both these Strolle and Schnitzler applications were commonly assigned to or under an obligation of assignment to the assignee of the current application at the times the respective inventions were made by the current applicants and their co-inventors.
The adaptive reemphasis circuit in the playback path of U.S. Pat. No. 5,113,262 performs an operation which can be referred to as "expansion of the dynamic range of luminance signal high frequencies", which is substantially the inverse of the "compression of the dynamic range of luminance signal high frequencies" operation carried out by the adaptive deemphasis circuit during recording. The adaptive deemphasis and adaptive remphasis procedures described in U.S. Pat. No. 5,113,262 are carried out in response to the high-frequency content of the luminance signal in static as well as dynamic portions of the image. The adaptive reemphasis circuit in the playback path includes circuitry for detecting the level of the high-frequency portion of the unfolded luminance signal, and circuitry for variably increasing the level of the high-frequency portion in response to the detected level. If the level of the high-frequency portion of the unfolded luminance signal is relatively high, then the level is boosted by the maximum amount; if the level is relatively low, then the level is boosted by the minimum amount.
Because when the level of the high-frequency portion of the luminance signal is high it is reduced by a maximum amount and when it is low it is reduced by a minimum amount, the level of the high-frequency portion is controlled to always be at about the same level. This deemphasized luminance signal is then folded, recorded, played back and unfolded. Each of these steps may introduce noise.
It is desirable to produce the highest quality image possible in a VCR with unfolding circuitry in the playback path, while maintaining backward compatibility with VCRs without the unfolding circuitry in the playback path. The compression of the dynamic range of luminance signal high frequencies done before recording a video tape should be compensated for by a complementary expansion of the dynamic range of luminance signal high frequencies reproduced from the video tape. This requires that the amount by which the high-frequency portion of the luminance signal is reduced in the record path substantially corresponds to the amount by which the high-frequency portion of the luminance signal is increased in the playback path. A solution described in U.S. Pat. No. 5,113,262, includes an adaptive reemphasis circuit in the playback path. The level of the high-frequency portion of the reproduced luminance after its unfolding is detected and used to control the high-frequency gain of the adaptive reemphasis circuit.
This circuit at times responds unfavorably to noise introduced in the record and playback process. If the introduced noise changes the detected level of the high-frequency portion, the reemphasis function is no longer substantially the inverse of the deemphasis function over the entire dynamic range of luminance high frequencies. Solutions to this problem are provided by reemphasis circuits that are constructed in accordance with aspects of the present invention.
Another solution to this problem is described in described in U.S. patent application Ser. No. 604,494 filed Jan. 25, 1993. A control signal, which is generated for controlling the deemphasis of the high-frequency portion of the luminance signal during its processing before being recorded, is encoded for recording on the video tape. During playback from the video tape, the control signal is reproduced and used for controlling the reemphasis of the high-frequency portion of the luminance signal. This avoids adaptive reemphasis circuitry that responds unfavorably to noise introduced in the record and playback process, but complicates the recording and playback procedures considerably.
Recording and playback procedures for transmitting auxiliary signals, such as a control signal for controlling the reemphasis of the high-frequency portion of the luminance signal during playback, are described in U.S. patent application Ser. No. 059,765 filed May 11, 1993 by Christopher H. Strolle, Werner F. Wedam, Jung-Wan Ko, Chandrakant B. Patel and Allen L. Limberg, entitled FREQUENCY-MULTIPLEXING FM LUMA SIGNAL WITH COLOR AND SECOND UNDER SIGNALS HAVING OVERLAPPING FREQUENCY SPECTRA, and assigned to Samsung Electronics Co., Ltd., pursuant to obligations of the inventors to so assign their invention at the time of its making. U.S. patent application Ser. No. 027,772 is a continuation-in-part of their U.S. patent application Ser. No. 531,070 filed May 31, 1990, entitled CHROMA CHANNEL ENCODED WITH AUXILIARY SIGNALS, and assigned to Samsung Electronics Co., Ltd., pursuant to obligations of the inventors to so assign their invention at the time of its making.
Accordingly, an improvement of the adaptive reemphasis circuitry described in U.S. Pat. No. 5,113,262 is desirable to have, which improvement does not respond unfavorably to noise introduced in the record and playback process, and which improvement does not require a control signal to be encoded for recording on the video tape or to be decoded when playing back from the video tape.